Examples

The following example creates the event source MySource if it doesn't already exist, and writes an entry to the event log MyNewLog.

Note

Starting with Windows Vista, you must run this application as an administrator.

#using <System.dll>
using namespace System;
using namespace System::Diagnostics;
using namespace System::Threading;
int main()
{
// Create the source, if it does not already exist.
if ( !EventLog::SourceExists( "MySource" ) )
{
//An event log source should not be created and immediately used.
//There is a latency time to enable the source, it should be created
//prior to executing the application that uses the source.
//Execute this sample a second time to use the new source.
EventLog::CreateEventSource( "MySource", "MyNewLog" );
Console::WriteLine( "CreatingEventSource" );
// The source is created. Exit the application to allow it to be registered.
return 0;
}
// Create an EventLog instance and assign its source.
EventLog^ myLog = gcnew EventLog;
myLog->Source = "MySource";
// Write an informational entry to the event log.
myLog->WriteEntry( "Writing to event log." );
}

using System;
using System.Diagnostics;
using System.Threading;
class MySample{
public static void Main(){
// Create the source, if it does not already exist.
if(!EventLog.SourceExists("MySource"))
{
//An event log source should not be created and immediately used.
//There is a latency time to enable the source, it should be created
//prior to executing the application that uses the source.
//Execute this sample a second time to use the new source.
EventLog.CreateEventSource("MySource", "MyNewLog");
Console.WriteLine("CreatedEventSource");
Console.WriteLine("Exiting, execute the application a second time to use the source.");
// The source is created. Exit the application to allow it to be registered.
return;
}
// Create an EventLog instance and assign its source.
EventLog myLog = new EventLog();
myLog.Source = "MySource";
// Write an informational entry to the event log.
myLog.WriteEntry("Writing to event log.");
}
}

Option Explicit
Option Strict
Imports System
Imports System.Diagnostics
Imports System.Threading
Class MySample
Public Shared Sub Main()
If Not EventLog.SourceExists("MySource") Then
' Create the source, if it does not already exist.
' An event log source should not be created and immediately used.
' There is a latency time to enable the source, it should be created
' prior to executing the application that uses the source.
' Execute this sample a second time to use the new source.
EventLog.CreateEventSource("MySource", "MyNewLog")
Console.WriteLine("CreatingEventSource")
'The source is created. Exit the application to allow it to be registered.
Return
End If
' Create an EventLog instance and assign its source.
Dim myLog As New EventLog()
myLog.Source = "MySource"
' Write an informational entry to the event log.
myLog.WriteEntry("Writing to event log.")
End Sub 'Main
End Class 'MySample

Remarks

EventLog lets you access or customize Windows event logs, which record information about important software or hardware events. Using EventLog, you can read from existing logs, write entries to logs, create or delete event sources, delete logs, and respond to log entries. You can also create new logs when creating an event source.

Important

This type implements the IDisposable interface. When you have finished using the type, you should dispose of it either directly or indirectly. To dispose of the type directly, call its Dispose method in a try/catch block. To dispose of it indirectly, use a language construct such as using (in C#) or Using (in Visual Basic). For more information, see the "Using an Object that Implements IDisposable" section in the IDisposable interface topic.

In addition to providing access to individual event logs and their entries, the EventLog class lets you access the collection of all event logs. You can use the static members of EventLog to delete logs, get log lists, create or delete a source, or determine if a computer already contains a particular source.

There are three default event logs: Application, System, and Security. A Security log is read-only. Other applications and services you install, such as Active Directory, might have additional event logs.

There are security considerations when using the EventLog class. EventLog requires EventLogPermission permissions for specific actions in the .NET Framework 2.0 and later versions, or full trust in the .NET Framework 1.0 and 1.1. We recommend that EventLogPermission not be granted to partially trusted code. You should never pass any event log object, including EventLogEntryCollection and EventLogEntry objects, to less trusted code. For example, creating an EventLog object, writing an entry, and then passing the EventLog object to partially trusted code can create a security issue, because the ability to read and write to the event log allows code to perform actions such as issuing event log messages in the name of another application.

Starting with Windows Vista, User Account Control (UAC) determines the credentials of a user. If you are a member of the Built-in Administrators group, you are assigned two run-time access tokens: a standard user access token and an administrator access token. By default, you are in the standard user role. To run the code that accesses the Security log, you must first elevate your credentials from standard user to administrator. You can do this when you start an application by opening the shortcut menu for the application (if you're using a mouse, right-click the application icon) and indicating that you want to run as an administrator.

You can use EventLog to create custom event logs that you can view through the server's Event Viewer. Use the RegisterDisplayName method to display a localized name for your event log in the Event Viewer. Use the ModifyOverflowPolicy method to configure the behavior of your event log when it reaches its maximum log size.

To read from an event log, specify the log name (Log property) and server computer name (MachineName property for the event log. If you don't specify the server computer name, the local computer, ".", is assumed. It's not necessary to specify the event source (Source property), because a source is required only for writing to logs. The Entries property is automatically populated with the event log's list of entries.

To write to an event log, specify or create an event source (Source property). You must have administrative credentials on the computer to create a new event source. The event source registers your application with the event log as a valid source of entries. You can use the event source to write to only one log at a time. The Source property can be any random string, but the name must be distinct from other sources on the computer. The event source is typically the name of the application or another identifying string. Trying to create a duplicate Source value throws an exception. However, a single event log can be associated with multiple sources.

If the event source for the event log associated with the EventLog instance doesn't exist, a new event source is created. To create an event source in Windows Vista and later or Windows Server 2003, you must have administrative credentials.

This requirement is because all event logs, including Security logs, must be searched to determine whether the event source is unique. Starting with Windows Vista, users do not have permission to access the Security log; therefore, a SecurityException is thrown.

Important

Creating or deleting an event source requires synchronization of the underlying code by using a named mutex. If a highly privileged application locks the named mutex, trying to create or delete an event source causes the application to stop responding until the lock is released. To help prevent this problem, never grant UnmanagedCode permission to untrusted code. In addition, UnmanagedCode permission potentially allows other permissions to be bypassed and should only be granted to highly trusted code.

Applications and services should write to the Application log or to a custom log. Device drivers should write to the System log. If you do not explicitly set the Log property, the event log defaults to the Application log.

Note

There is nothing to protect an application from writing as any registered source. If an application is granted Write permission, it can write events for any valid source registered on the computer.

Use the WriteEvent and WriteEntry methods to write events to an event log. You must specify an event source to write events; you must create and configure the event source before writing the first entry with the source.

Create the new event source during the installation of your application. This allows time for the operating system to refresh its list of registered event sources and their configuration. If the operating system hasn't refreshed its list of event sources, and you try to write an event with the new source, the write operation will fail. You can configure a new source by using an EventLogInstaller object or the CreateEventSource method. You must have administrative credentials on the computer to create a new event source.

Each source can write to only one event log at a time; however, your application can use multiple sources to write to multiple event logs. For example, your application might require multiple sources configured for different event logs or different resource files. To change the configuration details of an existing source, you must delete the source and then create it with the new configuration. If other applications or components use the existing source, create a new source with the updated configuration instead of deleting the existing source.

You can register the event source with localized resources for your event category and message strings. Your application can write event log entries by using resource identifiers instead of specifying the actual string values. Refer to the EventLogInstaller and EventSourceCreationData classes for more information about configuring your source with resource files.

If your application writes string values directly to the event log, you do not have to set the resource file properties for the source. The source must be configured either for writing localized entries or for writing direct strings. If your application writes entries using both resource identifiers and string values, you must register two separate sources. For example, configure one source with resource files, and then use that source in the WriteEvent method to write entries using resource identifiers to the event log. Then create a different source without resource files, and use that source in the WriteEntry method to write strings directly to the event log using that source.

When writing events, you must at least specify either a message string or the resource identifier for a message string. Other event properties are optional. Examples of optional event settings include the following:

You can set the EventLogEntryType to specify the icon that the Event Viewer displays for the entry.

You can specify a category identifier for the event, if your application uses categories for filtering the events.

You can attach binary data to your event entry if you want to associate additional information with a given event.

Important

Event logging consumes disk space, processor time, and other system resources. It is important to log only essential information. We recommend that you place event log calls in an error path, rather than in the main code path, so they don't adversely affect performance.

For a list of initial property values for an instance of EventLog, see the EventLog constructor.

Writes an entry with the given message text, application-defined event identifier, and application-defined category to the event log, using the specified registered event source. The category can be used by the Event Viewer to filter events in the log.

Writes an entry with the given message text, application-defined event identifier, and application-defined category to the event log (using the specified registered event source) and appends binary data to the message.